The present invention is directed to enhancing the quality of text and images generated by printing devices. More particularly, the present invention is directed to a method for improving image quality degraded by dot placement errors.
Ink deposition printers such as piezoelectric, acoustic, phase change, or thermal, conventionally employ at least one printhead from which droplets of ink are directed towards an imaging surface. The imaging surface may comprise an output sheet or an intermediate transfer surface from which ink is transferred to an output sheet. Typically, ink is stored within the printer in a liquid form and fed to the printhead. Droplets of ink are expelled in a raster fashion from jets (orifices or nozzles) in the printhead. In a solid ink printer, solid ink sticks are typically stored in an ink supply unit. Conventionally, the ink supply unit supplies molten ink to a reservoir by the melting of the solid ink sticks. The ink is maintained in a liquid state in the reservoir and fed to the printhead for propulsion upon demand, most normally through the use of piezo-electric transducers.
Ink deposition printers can provide very good image quality when dots can be placed reliably and accurately. However, when dot positioning (dot placement) is less accurate or reliable, there can be a noticeable reduction of image quality. There are a variety of factors that can contribute to dot positioning errors. One source of dot placement error results from inconsistencies and/or contaminants in the ink as well as the use of non standard inks. These may vary the flow rate in the print head, cause the ink to foul or clog nozzles in the print head, or change the thermal characteristics of the ink. Each of these can cause variations in the amount and the location of ink deposition. Additionally, design choices and/or manufacturing flaws can impact the rate ink flows through or discharges from the print head; thereby resulting in inconsistent and inaccurate drop placement. Another type of dot misplacement, which can result in a misplacement of as much as several pixels, depends on a particular dot pattern or firing frequency.
Dot misplacement can cause a variety of problems including noisy halftone output and degraded text and line quality. Another problem caused by dot misplacement manifests itself as severe color banding as a result of dot placement errors combined with jet to jet variation. The problem is most noticeable in dark color fills, especially in a uniform tertiary color that covers the page with approximately 33% secondary dots (red, green or blue) and 67% black dots. For example, it has been found that when red dots and black dots form certain patterns in a row or column (solid ink printers print dots in columns), dot misplacement can cause some of the magenta and yellow dots to overlap the black dots, resulting in large hue shifts and loss of saturation. This problem is exacerbated by jet to jet variation which will cause some rows (columns) to change more than others, resulting in severe bands, the appearance of which can range from brown to muddy yellow green to dark gray.